Conventionally, a rotation rate of an electric motor (such as a synchronous electric motor without position sensor) is controlled by adjusting a proportional integral (PI) of the rotation rate. By using this method, the rotation rate may vary according to a normal variation of a load, thus ensuring a correct output of a torque. However, when the variation of the load or a variation of the rotation rate is relatively large, using only the PI adjustment may lead to a low response or a large fluctuation of the rotation rate.